The controlled amplification of electromagnetic waves has many uses. For example, intelligence may be conveyed along a wave by amplifying electromagnetic wave characteristics so that the amplified wave can be perceived after transmission through a medium at a distance. Power amplifiers are commonly used in the communications field to increase the power of a modulated RF (radio frequency) signal which is then delivered to an antenna for transmission through the atmosphere. One widely used type of power amplifier is the switch-mode power amplifier, in which a transistor acts as a switch.
The output power of a switch-mode power amplifier is proportional to the resistance of the switch, however the switch may be implemented (for example, a MOS transistor acting in triode as a switched resistor). In many applications, especially cellular communications systems, it is important to control precisely the output power of the power amplifier. However, the resistance of the transistor, and thus the output power of the switch-mode power amplifier, varies depending upon the operating temperature of the device. Moreover, the output power of individual power amplifiers also varies from amplifier to amplifier as a result of process variations that occur during the fabrication of the devices. This is especially true when the power amplifier is fabricated as an integrated circuit using CMOS (complementary metallic oxide semiconductor) processes.
What is needed is a technique for compensating the output power of a power amplifier for temperature and process variations.